1) Field of the Invention
This invention relates to a process for the synthesis of a zeolite of the MTT type, the zeolite obtained and its use as catalyst and as selective organophilic adsorbent.
2) Background Art
The structure of zeolite MTT was established in 1985 by Rohrman and co-workers (Zeolites, 5, 352, 1985) and Wright and co-workers (J.C.S. Chem. Commun. 1117, 1985). It is a zeolite of orthorhombic symmetry (a=2.15 nm, b=1.11 nm and c=0.50 nm) which has 0.52.times.0.45 nm channels parallel to the c axis bounded by rings containing ten tetrahedra. In the other two crystallographic directions the zeolite exhibits a very dense lattice of rings containing six tetrahedra. These structural characteristics endow zeolites of MTT type with a unidirectional porosity and an excellent stability of the supporting structure.
Zeolites whose structure corresponds to the MTT type which are known so far are ZSM-23, EU-13, ISI-4 and KZ-1 (Meier and Olson, Atlas of Zeolite Structure Types, Butterworth, 1987). They are solids of interest for the catalytic conversion of hydrocarbons and of organic compounds and for adsorption.
The synthesis of zeolites of MTT type is described in a number of patents summarised in Table I. It is also described in the scientific literature by Parker and Bibby (Zeolites, 3, 8, 1983), Ernst et al. (Catal. Today, 3, 1, 1988) and Zones (Zeolites, 9, 458, 1989).
TABLE I ______________________________________ Applicant Priority Zeolite Patent No. Company year obtained Structurant ______________________________________ FR 2313980 Mobil 1975 ZSM 23 pyrrolidine EP 108486-B ICI 1982 EU 13 ammonium or phosphonium EP 125078-B Mobil 1983 ZSM 23 heptane-1,7- bis-trimethyl- ammonium EP 178846-A Mobil 1983 ZSM 23 heptane-1,7- bis-trimethyl- ammonium EP 220893-A Mobil 1985 ZSM 23 pyrrolidine GB 2190910-B BP 1986 ZSM 23 diisopro- panolamine GB 2202838-B ICI 1986 ZSM 23 octane-1,8- bis-trimethyl- ammonium EP 102497-B Res. 1982 ISI 4 ethylene Asso. glycol or Petrol. monoethanol- amine ______________________________________
The zeolites of MTT type are prepared by hydrothermal crystallisation of reactive alkali metal aluminosilicate gels. The syntheses are performed in the presence of a nitrogenous organic agent such as pyrrolidine, tertiary and quaternary alkylammoniums or diisopropanolamine. In Patent EP 102,497-B the organic agent employed is monoethanolamine or ethylene glycol.
At the end of the synthesis the organic agent is present trapped in the porosity of the zeolite and its removal, which is necessary to free the micropore volume of the zeolite, is performed by an oxidizing calcination at a temperature which is generally higher than 500.degree. C. This calcination stage presents a number of disadvantages because, in addition to its energy cost, it can entail a degradation of the crystalline structure and is accompanied by a release of nitrogen oxides which it is indispensable to treat for environmental reasons. In addition, the cost of the syntheses is augmented because the organic agent is generally the most costly component of the reaction mixture.
A process for the synthesis of a zeolite of MTT type which does not use any nitrogenous organic agent has now been found.